Thermo Sudoku
Digits must strictly increase along thermometer shapes
Easy
Thermo Sudoku Easy
Medium
Thermo Sudoku Medium
Hard
Thermo Sudoku Hard
Expert
Thermo Sudoku Expert
What is Thermo Sudoku?
Thermo Sudoku overlays thermometer shapes on the grid. Each thermometer has a bulb (circle) at one end and a tip at the other. The digits along any thermometer must strictly increase from the bulb to the tip. Combined with standard Sudoku constraints, this creates powerful chains of deductions — knowing the bulb is at least 1 and the tip is at most 9, you can calculate the range of valid values for each cell on the thermometer based on its position.
At a Glance
| Constraint type | Line Constraints |
| Typical givens | 20–28 |
| Difficulty rating | ★★★★☆ 4/5 |
| Avg. solve time — Easy | 10 min |
| Avg. solve time — Medium | 22 min |
| Avg. solve time — Hard | 45 min |
| Avg. solve time — Expert | 75 min |
How to Solve Thermo Sudoku
| Technique | What it does | Level |
|---|---|---|
| Range Restriction from Length | A thermometer with N cells must use N strictly increasing distinct values. The bulb is at least 1 and the tip is at most 9, giving a calculable valid range for each cell. | Beginner |
| Minimum/Maximum Bounding | The bulb value is ≤ 9 − (N − 1). The tip value is ≥ N. Apply these bounds immediately to prune all thermometer candidates. | Beginner |
| Overlapping Thermometer Deduction | Two thermometers sharing a cell give tighter combined bounds than either alone. | Intermediate |
| Forcing Chains via Thermometers | When a value is forced into a thermometer cell, all cells closer to the bulb must be smaller and all closer to the tip must be larger. | Advanced |
| Short vs. Long Thermometers | A 2-cell thermometer is simply a greater-than relationship. A 9-cell thermometer forces exactly 1–9 in order. | Intermediate |
Average Solve Times
Easy
10 min
Medium
22 min
Hard
45 min
Expert
75 min
Frequently Asked Questions
Which end is the bulb?
The circle (round end) is the bulb — digits increase from the bulb towards the tip.
Can a thermometer cross box boundaries?
Yes — thermometers can cross any boundary.
Must digits be strictly increasing?
Yes — strictly increasing means no two adjacent thermometer cells may share the same digit.